The inventive subject matter relates to power distribution systems and methods and, more particularly, to automatic transfer switch (ATS) apparatus and methods of operating the same.
Power distribution systems in industrial facilities, data centers and other applications commonly include at least one automatic transfer switch (ATS) that is configured to switch between alternative power sources. For example, an ATS may be used in a data center application to selectively connect a load to first and second utility feeds, or to selectively connect a load to a primary utility feed and a backup source, such as a local motor/generator set.
ATSs may have a variety of different operational characteristics. Some ATSs provide open transitions, which are “break before make” transitions in which the ATS breaks a connection between a load and a first power source before making a connection between the load and a second power source. Open transitions may be delayed, in which a sizeable delay is provided between disconnection of the load from the first source and connection of the load to the second source, which can reduce inrush current but can result in an undesirably long interruption in power. In an in-phase open transition, the ATS waits to transfer power until a point at which the first and second sources are substantially in phase.
Some ATSs support closed transitions, which are “make before break” transitions in which the second source is connected to the load before the first source is disconnected from the load. Such transitions typically are constrained to occur only when the first and second sources are sufficiently in sync (e.g., frequency, phase and voltage within a predetermined tolerance). Parallel connection of the first and second sources is typically limited to a very short period (e.g., 100 milliseconds) to prevent large fault currents that may trip circuit breakers or have other undesired effects.